Cyclotrons are circular accelerators with which charged particles may be accelerated, such as positive ions (protons, deuterons, helium nuclei, alpha particles, etc.) or negative ions (H−, D−, etc.), which are used i.a. for producing radioactive isotopes, for radiotherapy, or for experimental purposes. A cyclotron of the isochronous type essentially comprises:                an electromagnet comprising an upper pole and a lower pole, positioned symmetrically relatively to a middle plane, perpendicular to the central axis of the cyclotron, and separated by a gap provided for the circulation of charged particles, each of said poles comprising several sectors positioned so as to have an alternation of areas with a narrow gap commonly called <<hills>> and of areas with a wide gap commonly called <<valleys>>;        flux returns for closing said magnetic circuit;        a main induction coil for generating a essentially constant main induction field in the gap between said poles.        
An example of a cyclotron of the isochronous type is described in document BE1009669. In an isochronous cyclotron, the profile of the magnetic field should be such that the frequency of rotation of the particles is constant and independent of their energy. In order to compensate the relativistic mass increase of the particles, the average magnetic field should increase with the radius so as to ensure this isochronism condition. In order to describe this relationship, the field index is defined by the following relationship (1):
                    n        =                              R            B                    ⁢                                    ⅆ              B                                      ⅆ              R                                                          (        1        )            wherein dB/B and dR/R are respectively relative variations of the magnetic field B and of the radius with respect to the radius R.
The increase in the intensity of the magnetic field is effected according to a law given by equation (2):
                              B          ⁡                      (            R            )                          =                              B            0                    ⁢                                                    [                                  1                  -                                                                                                              (                          qB                          ]                                                0                                            ⁢                      R                                                                                      m                        i                                            ⁢                      c                                                                      )                            2                                                          (        2        )            wherein    B(R) is the average magnetic field around a circle of radius R;            B0 is the magnetic field at the center of the cyclotron;        q, the charge of the particle;        mi, the rest mass;        and c, the velocity of light.In the following of the text, mi will be considered in a first approximation as the mass of the particle m given by the product of the mass number A by the nucleon mass mN.        
In certain isochronous cyclotrons, the sectors are machined so as to accelerate one type of particle with a well specified <<charge over mass>> q/m. For example a cyclotron for which the sectors are machined for accelerating particles with a <<charge-over-mass>> ratio q/m=½ may accelerate alpha particles, deuterons D−, HH+, 6Li3+, 10B5+ or 12C6+ or other particles with the same q/m ratio=½. The acceleration of another type of particles with a ratio of q/m=1 requires the use of another cyclotron for which the sectors are machined for acceleration of this type of particles.
It is nevertheless possible in an isochronous cyclotron to pass from a first magnetic field profile allowing acceleration of a first type of particles to a second magnetic field profile for accelerating a second type of particles, wherein by means of concentric annular coils for magnetic field correction, positioned at the surface of the poles according to a well specified distribution, each of said concentric coils being connected to a specific current generator in order to induce the required additional magnetic field. An example of such a device is described in document U.S. Pat. No. 3,789,355. Nevertheless, the number of coils each connected to a specific current generator, the distribution of these coils and the current to be applied in each coil for obtaining the desired magnetic field, complicate the making and the use of this kind of cyclotrons.
Other cyclotrons, such as the Cyclone 18/9 of IBA, have been designed so as to be able to accelerate different types of ions characterized by their different <<charge-over-mass>> ratio q/m. The cyclone 18/9 may accelerate protons (q/m=1) to an energy of 18 MeV, deuterons (q/m=½) to an energy of 9 MeV. The isochronous magnetic field profile has to be adapted depending on the type of particles to be accelerated. FIG. 1 shows the profiles of average magnetic fields <B> versus the average radius <R> of the particle in the cyclotron for accelerating particles with a ratio q/m equal to 1 and particles with a <<charge-over-mass>> ratio q/m equal to a ½. By virtue of equation (2), for a same average radius of the particle in the cyclotron, the average magnetic field should be larger for accelerating protons than for accelerating deuterons. In the case of the Cyclone 18/9 and Cyclone 30/15 of IBA, a mechanical means supports ferromagnetic plates which extend, in two opposite valleys, from an area close to the center of the cyclotron to the periphery of the cyclotron. For accelerating protons, said mechanical means positions said ferromagnetic plates in proximity to the middle plane of the cyclotron in order to provide an additional field giving the possibility of obtaining the required isochronous magnetic field profile. For accelerating deuterons requiring a different average magnetic field profile according to the average radius, said ferromagnetic plates are moved away relatively to the middle plane so as to decrease or suppress the intensity of the additional magnetic field and obtain the required isochronous magnetic field profile for accelerating deuterons.
In the case of low energy cyclotrons, the corrections to be carried out on the magnetic field for passing from one magnetic field profile intended for accelerating particles with a ratio q/m=½ to a magnetic field profile intended for accelerating particles with a ratio q/m=1 do not require the application of an additional magnetic field which is too large. In a first approximation, it is considered that for accelerating protons, the profile of the average magnetic field versus the average radius varies by increasing by about 1% per <<step>> of 10 MeV. The profile of the average magnetic field versus the average radius increases by about 0.5% per step of 10 MeV for the case of deuterons. For example, for a 10/5 cyclotron capable of accelerating protons to an energy of 10 MeV and deuterons to an energy of 5 MeV, the variation of the average magnetic field from the center of the cyclotron to the end of the poles is of 1% for the proton and 0.25% for the deuteron. In this case, said ferromagnetic plates as used in the Cyclone 18/9 and Cyclone 30/15 are sufficient for producing the additional magnetic field required for accelerating protons. If it is desired to design a cyclotron capable of accelerating protons to 70 MeV and deuterons to 35 MeV, the variation of the profile of the average magnetic field from the center of the cyclotron towards the end of the poles would have to be about 7% for accelerating protons and 1.75% for accelerating deuterons. For accelerating deuterons, the variation of the profile of the average magnetic field versus the average radius only requires adequate machining of the sectors, i.e. an azimuthal widening of the hills in proximity to the ends of the poles. If this solution for accelerating deuterons poses very little problems as regards manufacturing, on the other hand for accelerating protons, said ferromagnetic plates should be able to reduce sufficient additional magnetic field for obtaining the desired average magnetic field profile versus the average radius. With said ferromagnetic plates, it is not possible to produce a sufficiently large additional magnetic field for ensuring isochronism. On the other hand, the volume comprised between two hills does not allow azimuthal widening of said ferromagnetic plates with the purpose of generating an additional magnetic field.
The document <<Magnetic field design and calculation for the IBA C70 cyclotron>> S. Zaremba et al., Cyclotrons and their applications 2007, Eighteenth International Conference, pages 75-77, describes the development of an isochronous cyclotron called C70 or Cyclone 70, capable of accelerating 4 types of particles: protons (q/m=1) and alpha particles (q/m=½) to an energy of 70 MeV, as well as deuterons (q/m=½) and HH+ (q/m=½) to an energy of 35 MeV. This document explains the different solutions which have been contemplated in order to obtain a cyclotron which may operate according to two different isochronous magnetic fields so as to simulate a type of particles with a desired q/m ratio. This cyclotron C70 comprises hills divided into three superposed portions and parallel to the middle plane:                a first portion away from the middle plane forming the basis of the hill;        a second central portion forming a pole around which are wound correction coils with a specific distribution and;        a third portion, the closest to the middle plane, being a plate for shielding the correction coils.        
This configuration of hills is nevertheless complicated and requires very accurate alignment of said three portions as well as very accurate distribution of the coils. As an intensive vacuum is required inside the cyclotron, in particular for accelerating negatively charged particles, the assembly should be able to support significant variations in pressure, without this producing misadjustment of the different parts. Also, during the application of vacuum to the cyclotron, degassing problems at the correction coils may occur, the latter being confined between the base of the hill and the shielding plate. Finally, it is necessary to optimize the thickness of the shielding plate so that the magnetic flux fraction useful for accelerating the particles in the gap is sufficient while retaining some mechanical rigidity of said plate.
The object of the present invention is to provide a cyclotron capable of accelerating types of particles with different <<charge-over-mass>> q/m ratios, not having the drawbacks of the prior art.
Another object of the present invention is to provide a cyclotron with means for correcting the profile of the magnetic field according to the q/m ratio of the type of particles to be accelerated, said means allowing a simpler embodiment than the means of the prior art.
Another object of the present invention is to provide a cyclotron with a means for correcting the profile of the magnetic field according to the q/m ratio of the type of particles to be accelerated, said means may produce a sufficient additional magnetic field in the case of medium to high energy cyclotrons.
Another object of the present invention is to provide a cyclotron with a means for correcting the magnetic field profile not perturbing the internal vacuum of the cyclotron.